Forage quality for U.S. pollinators largely remained stable over a 30-year period, but declines in key areas could spell trouble for honey bees, USDA researchers found.

More than a third of food crops require pollination, so recent health problems seen among honey bees and native pollinators are concerning for farmers.

Poor nutrition — along with pests, pathogens and pesticides — is considered one of the prime factors adversely affecting pollinator health, said Dan Hellerstein, lead author of the USDA Economic Research Service’s study, “Land Use, Land Cover, and Pollinator Health: A Review and Trend Analysis.”

When the suitability of pollinator forage is diminished, “you will likely have more impacts from the other stressors in bees,” Hellerstein said.

While anecdotal evidence points to a decline in U.S. pollinator forage quality, Hellerstein and his co-researchers wanted to see if actual data backed up that theory.

“Can we quantify this notion of a trend?” he said.

The researchers analyzed nearly 1 million geographic data points collected across the U.S. between 1982 and 2012 as part of USDA’s Natural Resources Inventory, scoring them based on a “forage suitability index.”

In the aggregate, the results were fairly benign: Forage suitability was unchanged on 75 percent of the acreage in the 30-year period, while it declined on 11 percent and improved on 14 percent.

However, the average forage suitability fell between 2002 and 2012 after increasing over the previous two decades.

The drop was particularly steep in the parts of North Dakota and South Dakota where beekeepers allow hives to recover after they travel across the U.S., pollinating various crops.

This finding is significant because honey bees that build up their strength in those areas over the summer will presumably be better equipped to survive the winter, said Hellerstein.

Diminished forage quality in the Dakotas is associated with fewer acres devoted to crops with high suitability scores — such as sunflowers and berries — and an increase in corn and soybeans, which aren’t as pollinator friendly.

“In the Dakotas, it seems to be the advancement of the corn and soybean belt,” said Amélie Davis, an assistant professor at Miami University who co-authored the study.

These key summer locations also experienced a marked decline in quality forage acreage committed to USDA’s Conservation Reserve Program, which pays farmers to convert land from crops to native plant species.

The reduction of CRP land is typically associated with higher commodity crop prices persuading growers to cultivate marginal lands, as well as a lower cap on such acreage under the 2014 Farm Bill.

In some areas, such as parts of Texas, forage suitability has improved as farmers switched acreage from crops to pasture, said David Smith, an ERS economist who co-authored the study.

Such changes in land use affect not only honey bees but native pollinators that also perform a valuable service for growers, Davis said.

Forage quality is one part of a broader equation that determines pollinator health, she said. “It’s the synergy of all those issues that’s affecting them so strongly.”